What are the appropriate lab studies for a patient presenting with pneumonitis?

Appropriate Laboratory Studies for Pneumonitis

For patients presenting with pneumonitis, appropriate laboratory studies should include chest radiography, complete blood count, basic blood chemistry, arterial blood gas or pulse oximetry, and C-reactive protein, with additional microbiological investigations guided by disease severity. 1, 2

Initial Laboratory Evaluation

Basic Laboratory Studies (All Patients)

• Chest radiograph (essential for confirming diagnosis and identifying complications) 1, 2
• Complete blood count with differential 1, 2
• Basic blood chemistry:
  • Electrolytes (sodium, potassium)
  • Renal function (creatinine, urea nitrogen)
  • Liver function tests (aminotransferases)
  • C-reactive protein (CRP) when available 1
• Oxygenation assessment (arterial blood gas or pulse oximetry) 1, 2

Microbiological Investigations

The extent of microbiological workup should be determined by pneumonitis severity 1:

Mild Pneumonitis (Outpatient Management)

• Microbiological investigations generally not recommended routinely 1
• Consider sputum examination if:
  • Patient does not respond to empiric therapy 1
  • Tuberculosis is suspected (persistent productive cough with malaise, weight loss, night sweats) 1

Moderate to Severe Pneumonitis (Hospitalized Patients)

• Blood cultures (preferably before antibiotic administration) 1, 2
• Sputum samples for:
  • Gram stain (for patients with severe pneumonitis or complications) 1
  • Culture and sensitivity testing (if purulent sputum available and no prior antibiotics) 1
• Legionella urinary antigen testing (for severe pneumonitis or when epidemiologically suspected) 1, 2
• Pneumococcal antigen testing (if available) 1, 2
• Paired serological tests for:
  • Patients with severe pneumonitis
  • Patients unresponsive to β-lactam antibiotics
  • Cases with specific epidemiological risk factors 1

Additional Tests for Specific Situations

• Thoracentesis for significant pleural effusions (≥10mm thickness on lateral decubitus film) 1, 2
• Bronchoscopy with protected specimen brush (PSB) or bronchoalveolar lavage (BAL) for:
  • Severely ill patients not responding to empiric therapy
  • Suspected non-infectious pneumonitis requiring tissue diagnosis 1, 2
• HIV testing (after informed consent) in patients with risk factors or aged 15-54 years admitted for pneumonia 1

Advanced Imaging

• Consider chest CT when:
  • Clinical findings don't match radiographic results
  • High clinical suspicion despite negative chest X-ray
  • Patients show signs of severe infection 2

Pitfalls and Caveats

• Chest radiography has limited sensitivity for early or subtle infiltrates, particularly in elderly patients 2
• Blood cultures have low yield (7-11%) but should still be performed in hospitalized patients 1, 3, 4
• Sputum samples should be collected before antibiotic administration and transported rapidly to the laboratory 1
• Serologic testing and cold agglutinin measurements are not useful for initial evaluation but may help with retrospective diagnosis 1
• For suspected hypersensitivity pneumonitis, additional testing for serum precipitating antibodies (IgG) and bronchoalveolar lavage showing suppressor cytotoxic lymphocytosis may be helpful 5

The laboratory approach should be tailored to disease severity, with more extensive testing for moderate to severe cases requiring hospitalization, while limiting unnecessary testing in mild cases that can be managed as outpatients.